1. Field of the Invention
The present invention relates to fuel injection control for an internal combustion engine, particularly asynchronous injection control at the time of restoration from a fuel cut mode.
2. Description of the Background Art
When a spark ignition type engine is shifted from a steady operation state to an accelerating operation state, fuel injection that is not in synchronization with the engine revolution for the purpose of increasing fuel (hereinafter, referred to as asynchronous injection) is conducted aside from fuel injection at every engine revolution (hereinafter, referred to as synchronous injection). For example, in the case of a multicylinder engine mounted on a vehicle, fuel is injected in synchronization into the intake port by an injector from the later stage of the exhaust stroke to the intake stroke for each cylinder in a steady operation state. When an action is made by the driver of the vehicle to open the throttle valve, asynchronous injection is effected immediately for all the cylinders. Accordingly, the fuel supply to all the cylinders is increased without delay corresponding to the increase of the intake air to avoid dilution of the air-fuel mixture and to avoid degradation in the vehicle acceleration response. Thus, the drivability can be improved.
In such asynchronous injection, a constant amount of fuel is injected by sensing the accelerating operation state, irrespective of the crank angle, to avoid degradation in the acceleration response. However, there is the problem that deviation from the stoichiometric air-fuel ratio may occur as the number of times of asynchronous injection increases since the stoichiometric air-fuel ratio is not taken into account as in synchronous injection. Japanese Patent Laying-Open No. 08-158920 discloses a correcting control device during the transition period of electronic fuel injection that can maintain the stoichiometric air-fuel ratio even in asynchronous injection. This correcting control device for the transition period of electronic fuel injection estimates the intake pipe pressure of a preread crank angle from the time of calculation to the average timing of each cylinder taking in fuel based on pressure changes in the past in the intake pipe when fuel injection is to be effected at one time for all the cylinders. The correcting control device accommodates synchronous injection effecting fuel injection of one cycle taking into consideration the stoichiometric air-fuel ratio based on the estimated pressure, and asynchronous fuel injection effecting injection of fuel that runs short in synchronous injection at the time of abrupt acceleration. The amount of fuel injection for the asynchronous fuel injection is obtained taking into consideration the stoichiometric air-fuel ratio by estimating the intake pipe pressure, likewise synchronous injection. The maximum value of the amount of fuel to be injected in one cycle is limited in synchronous injection and asynchronous injection.
In accordance with this correcting control device, the fuel injection amount in asynchronous fuel injection is obtained taking into account the stoichiometric air-fuel ratio by estimating the intake pipe pressure, likewise synchronous injection, so that deviation from the stoichiometric air-fuel ratio, even when the asynchronous fuel injection amount increases, can be suppressed. Further, by limiting the maximum value of the amount of fuel to be injected in one cycle in synchronous injection and asynchronous injection, excessive correction can be suppressed.
The exhaust system of an engine is generally provided with a catalytic converter to purify specific components in the exhaust gas. A three-way catalytic converter is used extensively for such a catalytic converter to oxidize carbon monoxide (CO) and unburned hydrocarbon (HC) and to reduce nitrogen oxide (NOx), which are the specific three components in exhaust gas, for conversion into carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2), respectively.
The purifying property by the three-way catalytic converter depends upon the air-fuel ratio of the air-fuel mixture formed in the combustion chamber. The three-way catalytic converter functions most effectively when the air-fuel ratio is in the vicinity of the stoichiometric air-fuel ratio. This is due to the fact that, if the air-fuel ratio is lean and the amount of oxygen in the exhaust gas is large, oxidation becomes active whereas reduction becomes inactive, and if the air-fuel ratio is rich and the amount of oxygen in the exhaust gas is small, reduction becomes active whereas oxidation becomes inactive, such that all the three components set forth above cannot be purified favorably. Therefore, an engine with a three-way catalytic converter has an output linear type oxygen sensor provided at the exhaust manifold such that the air-fuel mixture in the combustion chamber is feedback-controlled to the stoichiometric air-fuel ratio based on the oxygen concentration measured by the oxygen sensor. In other words, when the air-fuel ratio is lean and the amount of oxygen in the exhaust gas is large, the reduction action becomes inactive, which means that the action of reducing nitrogen oxide (NOx) is deteriorated to degrade the NOx purifying function.
The vehicle employs the control to suppress fuel supply during deceleration in order to improve the fuel economy, i.e. fuel-cut control. This fuel cut control aims to improve the fuel economy by reducing fuel supply to the engine as much as possible in a range that does not spoil the running performance and riding comfort. In general, fuel supply is suppressed when the engine speed falls within a predetermined range (equal to or higher than the fuel-cut speed) during deceleration in which the engine takes an idling state. Specifically, when the throttle valve is closed during running and the engine speed is equal to or higher than the fuel cut speed, supply of fuel is ceased. When the engine speed is reduced to arrive at the restoration speed that defines the lower limit of the range (fuel cut restoration speed), fuel supply is resumed.
Since fuel injection is suppressed during fuel-cut control, the air-fuel ratio is rendered lean, and the NOx purifying function is degraded. If fuel is injected upon restoration from the fuel-cut state, the NOx cannot be purified sufficiently since the NOx purifying function by the three-way catalytic converter is degraded. Therefore, the asynchronous injection based on emission request set forth above is executed.
If the fuel injection amount in asynchronous injection is excessive, the air-fuel ratio will become too rich to cause backfire. If the fuel injection amount is too low, a sufficiently rich atmosphere cannot be achieved in the three-way catalytic converter, leading to the problem that the NOx purifying function cannot be improved.
These problems, however, are not recognized in the aforementioned Japanese Patent Laying-Open No. 08-158920. Although the control device disclosed in this publication restricts the amount of fuel injection per cycle in order to suppress excessive correction, the problems set forth above are not addressed.